The Loyalhanna is a sparsely fossiliferous, distinctively cross-bedded, sandy calcarenite and calcareous sandstone. It occurs along the outcrop belt and in the subsurface of Pennsylvania and West Virginia where it is less than 100 ft (30 m) thick. In West Virginia, the Loyalhanna has produced more than 200 million bbl of oil, accounting for much of the state’s total oil production.
In thin section, Loyalhanna lithologies consist of grainstones with varying proportions of quartz, ooids, fossils, and peloids. Quartz was introduced from a northern source area, and ooids were transported by currents from shoals to the south. Skeletal grains include crinoids, bryozoans, ostracodes, and foraminifera. Peloids are mainly micritized ooids and fossils.
The environmental setting was a high-energy, sublittoral sand flat that extended along the northern coastline of the Mississippian embayment into the central Appalachians. The sediment was deposited as low-relief sand waves with an internal structure of avalanche-style cross-bedding. As the sand built up, the cross-bedded units were capped by horizontal beds. Cross-bedding indicates that sand waves migrated to the northeast under the influence of longshore currents. Minor fluctuations in sea level and sedimentation rate produced a widespread blanket sand.
In the producing areas of West Virginia, porosity development has resulted from early dolomitization. Intercrystalline and moldic porosity is good, typically reaching 15-25%. However, this combination of porosities, which is closely related to original sedimentary textures, has led to only fair permeability and a fair recovery efficiency for the unit.